RS 232 file transfer is a simple function in PC using Hyperterminal. Set the data speed appropriately as per the UART interface speed of the SDIO controller that you are using. And use the transfer file option in Hyper terminal. Most of the times this is a seamless and hassle-free way of file transfer through UART. Just make sure whether or not your SDIO-UART controller needs flow control or not. Appropriately enable or disable the flow control in Hyperterminal.
Hope this helps.